The present invention relates to steel strapping.
Steel strapping is formed by slitting cold rolled steel strip into a required width and is used in a variety of applications that require a range of properties. Generally, the properties which must be considered when producing strapping are tensile strength, ductility, notch properties and work hardening. These properties are dependent on the composition of the steel and the heat treatment processes applied to the strapping.
The minimum tensile strength of steel strapping varies between 500 and 1250 MPa.
Strapping having tensile strengths in the range 500 to 800 MPa is manufactured and sold by the applicant as xe2x80x98standardxe2x80x99 strapping.
Strapping having tensile strengths in excess of 800 MPa is manufactured and sold by the applicant as xe2x80x98superxe2x80x99 strapping.
Standard strapping is generally formed from low carbon steels and may be used in its cold rolled and slit form without heat treatment in applications requiring moderate strength levels, for example in the securing of cardboard cartons to pallets. In some instances standard strapping is formed from medium carbon steels and is subjected to a stress relief annealing treatment or a blueing heat treatment in order to improve ductility.
Super strapping is generally formed from medium carbon steels and the strapping is subjected to heat treatment to provide required properties. Super strapping is used in heavy duty applications requiring medium to high tensile strength and good ductility, notch properties and work hardening. Uses include unitising of steel pipe into bundles, fastening heavy loads to pallets and containing high-density wool and cotton bales.
The basic production steps for super strapping are as follows:
(a) hot rolling steel slabs to form strip;
(b) cold rolling strip to a required strip thickness;
(c) slitting the strip to form strapping;
(d) heat treating the strapping to produce target mechanical properties and microstructure.
U.S. Pat. No. 4,816,090 of the applicant discloses a heat treatment process for steel strapping that comprises:
(a) rapidly heating the strapping to the dual phase temperature range using induction heating, with little or no soaking, and
(b) rapidly cooling the strapping to form a microstructure comprising a matrix of recovery annealed cold worked ferrite containing martensite and carbides dispersed throughout the matrix.
The term xe2x80x9cdual phasexe2x80x9d as used herein is understood to mean the phase equilibrium region where austenite and ferrite phases co-exist.
The U.S. patent discloses a heat treated steel strapping which is characterised by a microstructure comprising a matrix of recovery annealed cold worked ferrite containing martensite and carbides dispersed through the matrix.
The U.S. patent discloses that the composition of the steel preferably comprises less than 0.2 wt % C and is characterised by alloying elements that form precipitates which retard recrystallisation during the heat treatment process.
The U.S. patent discloses that titanium and niobium are preferred alloying elements.
The U.S. patent discloses that titanium may be present in the range 0.06-0.15 wt % and preferably 0.08 wt % and niobium may be present in the range 0.02 to 0.05 wt % and preferably about 0.04 wt %.
The applicant has manufactured commercially in Australia steel strapping disclosed in the U.S. patent by hot rolling steel slabs to form strip, typically 2.0 mm thick, cold rolling the strip, typically to a final strip thickness of 0.8 mm, slitting the strip to form strapping, and thereafter heat treating the strapping in an induction heating unit.
The applicant has encountered severe problems in the hot rolling step, with the result that the yield has been very low.
The applicant has determined that the problems were due to titanium in the steel causing inconsistent recrystallisation behaviour in the hot strip mill.
Titanium is an important element in the steel composition. Titanium is important because it forms precipitates of titanium carbides and, as noted above, these precipitates retard recrystallisation process during the heat treatment process. This, together with the rapid heating cycle in the heat treatment process, causes the retention of considerable cold work in the microstructure resulting in a substantial contribution to strength of the strapping.
As noted above, the U.S. patent discloses a titanium range of 0.06-0.15 wt %. The severe hot rolling problems that were encountered by the applicant were encountered when hot rolling steel having a titanium concentration of 0.065 wt %, ie towards the lower end of the range. This steel is referred to hereinafter as XAK15 feed steel.
The applicant has recognised that reducing or removing titanium altogether from the steel strapping composition, whilst overcoming the hot strip mill problems, would have a significant adverse impact on the mechanical properties, particularly tensile strength, of the strapping.
The applicant has carried out research project and has found that the above-described hot rolling problem could be overcome by using steel having the following composition, in wt %.
Specifically, the applicant has found that the above-described steel composition is not subject to the severe hot rolling problems encountered by XAK15 feed steel.
The applicant has found also that strapping manufactured from the above-described steel composition has mechanical properties, such as tensile strength, that are at least comparable to those of strapping made from XAK15 feed steel.
In particular, the applicant has found that at least comparable mechanical properties could be obtained by manufacturing strapping by hot rolling, cold rolling, and thereafter heat treating the steel composition to form a microstructure comprising a matrix of recovery-annealed cold worked ferrite containing martensite and carbides dispersed through the matrix.
The applicant believes that the addition of V to the steel composition and the reduction in the concentration of Ti in the composition (compared to the Ti concentration in XAK15 feed steel) prevented hot roll formability problems in the hot strip mill and that the addition of V resulted in good mechanical properties. In particular, the applicant believes that the addition of V and the reduction in the concentration of Ti made it possible to achieve consistent recrystallisation of steel in the hot strip mill and, more particularly, recrystallisation of steel in each stand of the hot strip mill Preferably the microstructure includes up to 15 vol % carbides.
Preferably the microstructure includes 5-10 vol % martensite.
Accordingly, the present invention provides a steel strapping having the above-described composition and microstructure.
Preferably the Ti concentration in the composition is 0.03 to 0.04 wt %.
Preferably the Mn concentration in the composition is 0.8 to 1.2 wt %.
Preferably the Al concentration in the composition is 0.015 to 0.08 wt %.
Preferably the tensile strength of the strapping is at least 900 Mpa.
Preferably the tensile strength of the strapping is at least 100 Mpa.
In addition, according to the present invention there is provided a method of manufacturing steel strapping which includes the steps of:
(a) hot rolling steel slabs by passing the slabs through a series of rolling mill stands in a hot strip mill and reducing the slab thickness to form steel strip;
(b) cold rolling the steel strip in a cold rolling mill and reducing the thickness of the strip; and
(c) heat treating the steel strip to produce steel strapping, and
the method being characterised by selecting the hot rolling conditions and the steel slab composition so that there is recrystallisation of steel in each stand of the hot rolling mill.
The above-described method is based on the realisation that hot rolled steel having required mechanical properties could be produced on a consistent basis, i.e. at high yields, by ensuring that there is at least some recrystallisation of steel in each mill stand.
Preferably heat treatment step (c) includes:
(i) rapidly heating the strapping to the dual phase temperature range using induction heating, with little or no soaking, and
(ii) rapidly cooling the strapping.
Preferably the method produces strapping having a microstructure comprising a matrix of recovery annealed cold worked ferrite containing martensite and carbides dispersed throughout the matrix.
Preferably the method further includes a step of slitting the cold rolled steel strip prior to heat treatment step (c).
The above-described steel composition is suitable for use in the above-described method.